Governments, prisons, military bases, and other secure facilities have an interest in securing ports of entry, borders, and boundaries from people and goods that are prohibited or that threaten their citizens, operations, infrastructure, resources or food supply. Clandestine underground tunnels, however, provide a means of surreptitiously crossing national borders and other security boundaries. For example, in the Unites States, a new border tunnel has historically been discovered every 3-6 months. In November of 2003 in Calexico, Calif., U.S. Department of Homeland Security officials uncovered a cross-border tunnel about 15 feet underground just 200 yards east of a similar tunnel they had unearthed two months earlier. Both tunnels had sophisticated lighting and ventilation systems. In addition to smuggling prohibited goods and humans across political boundaries, clandestine tunneling is historically a means by which detainees have successfully escaped imprisonment. A noteworthy instance of this was the July 2015 escape of international drug kingpin Joaquin “El Chapo” Guzman from a Mexican prison through a mile-long tunnel dug beneath his prison cell. Detection of clandestine tunnels and tunneling activity is therefore a high priority for security on scales ranging from national borders to facility boundaries. Many parties, including the U.S. Department of Homeland Security, have shown considerable interest in sensors and sensor systems that can detect and locate existing tunnels or new tunneling activity, or both.
Existing approaches to tunnel detection can be broadly categorized as either active or passive. Active approaches, such as ground penetrating radar (GPR) or seismic interrogation methods, generally examine the response of the subsurface environment to a signal from an active source. The strength, travel time, and phase of returned signals are interpreted via geophysical inversion techniques to create a 3D image of the structure of some property of the subsurface soil or rock, in hopes of identifying anomalies that may represent tunnels. In contrast, passive approaches attempt to detect physical signals generated by tunneling and tunnel use. Such signals include the seismic and acoustic energy generated by tunneling operations, acoustic emissions from tunnel use, or the electromagnetic signature of lighting and other equipment used in tunnels. Gravity gradiometers may attempt to identify minute gravitational anomalies associated with tunnels, and still more indirectly, coherent change detection using synthetic aperture radar (SAR) might detect ground subsidence due to underlying tunneling activity.
None of these methods is appropriate for detecting all tunnels in all geologic and cultural settings. Challenges to the effectiveness of active methods include: (a) they are difficult to apply clandestinely, (b) signal penetration and imaging resolution diminish with depth of investigation, (c) urban clutter and cultural noise can interfere or interact with interrogation signals to confound interpretation, (d) geophysical inversion of data generally imposes a high computational burden and cannot be performed real-time, and (e) subsurface structural anomalies identified can be caused by many things other than tunnels. In addition to some of the weaknesses of active methods, passive methods have their own set of challenges: cultural noise may interfere with passive signals, their strength generally diminishes as tunnel depth increases, and processing and interpretation of passive monitoring data is generally resource intensive and confounded by uncertainty and heterogeneity in the signal transmission characteristics of the subsurface medium.
The effectiveness of any given technique, both active and passive, also varies tremendously as a function of the geologic setting. Electrically conductive soils diffuse and absorb radar and other electromagnetic signals, weak or compressible soils absorb and attenuate seismic and acoustic energy, fractures and other structural variations reflect and refract propagating waves, etc. Practitioners in the art therefore widely recognize that no universal technological solution is yet available for detecting clandestine tunnels and tunneling activity.